1. Field of the Invention:
This invention relates generally to electric power systems, and more particularly the invention relates to power systems utilizing solar radiation to energize photovoltaic cells and to semiconductor PN junction devices used in photovoltaic cells.
2. Prior Art:
Solar radiation is recognized as a potential source for large amounts of energy, if properly harnessed. Historically, various heat transfer mechanisms have been devised for converting solar radiation into heat energy. In more recent years considerable effort has been directed to the conversion of solar energy to electrical energy through use of solar or photovoltaic cells. Such devices are employed in space applications, for example.
The photovoltaic cell comprises single crystalline silicon material in which a PN junction is formed by the selective introduction of elemental dopants into the semiconductor body. Doping techniques such as diffusion and ion implantation are well known in semiconductor processing technology.
In operation of the photovoltaic cell, a potential difference exists at the PN junction of the semiconductor cell due to the diffusion of electrical carriers, holes and electrons, across the PN junction which are then captured by majority carriers of the new region. By exposing the semiconductor cell to solar radiation, incident radiation is absorbed within the semiconductor body and will create electron-hole pairs or carriers which can be separated by the PN junction and made available to energize an external circuit. Only radiation or photons having an energy level of approximately 1.12 electron volts or higher can create an electron-hole pair in silicon. Such photons have a wavelength of 1.11 microns or shorter. Photons of greater wavelength having lesser energy may be absorbed by the cell as heat, and the excess energy of the shorter wavelength photons will be wasted as heat, also. Due to only a percentage of solar radiation (approximately 45% in silicon) being available for energy conversion and since the maximum power of a silicon photovoltaic cell is delivered at about one-half volt rather than 1.12 volts, maximum energy conversion without concentration of radiation is about 22%. However, in practice, other losses reduce this to about 10% in conventional photovoltaic cells.
The most widely employed of conventional photovoltaic cells is the planar junction device introduced by Chapin and Fuller at the Bell Telephone Laboratories in the mid-1950's. In such devices a PN junction is formed near a radiation receiving surface of a semiconductor body. Metallic electrode fingers are placed on the surface of the semiconductor body to form a current collection grid for the cell. Due to shadow loss from the collection grid on the radiation receiving surface and due to series resistance losses in the cells, the intrinsic efficiency of planar junction devices is substantially less than 20%.
The interdigitated back contact cell developed by Lammert and Schwartz of Purdue University eliminates the shadow loss by providing alternating P & N type regions on the back surface of a semiconductor body with the P regions connected in parallel and the N regions connected in parallel. The interdigitated contacts reduce series resistance losses, also. In this cell a silicon oxide layer is provided on the top surface to minimize the hole-electron recombination at the surface. Limitations of the cell include not only the more complex semiconductor processing in fabricating the device but also difficulty in optically matching the cell to the outside world.
Another prior art device is the vertical multi-junction cell proposed by Sater of NASA. This device is fabricated from a stack of semiconductor wafers having alternating N and P type conductivity and in which a thin aluminum layer is provided between wafers for adhesion purposes. The stack is then sliced to provide a cell with alternating P & N semiconductor regions, each separated by a thin aluminum layer. This device is characterized by a high surface recombination of holes and electrons because of the difficulty of surface passivation due to the presence of the aluminum material. Further, uniformity of the semiconductor regions is difficult to maintain due to the plurality of semiconductor wafers used in forming the stack.
Several embodiments of a monolithic photovoltaic semiconductor device are disclosed in U.S. Pat. No. 3,994,012 to Warner which utilize a plurality of series connected PN junctions in the monolithic body. One major limitation of the Warner devices is the involved and complex processing including several high temperature steps in fabricating the device. Additionally, the necessary electrical isolation between adjacent PN junction elements is difficult to achieve without the use of additional "bucking" or isolating PN junctions. Further, the Warner device utilizes a metal layer on the illuminated surface thereby creating a shadow loss in device operation.